Micro-electro-mechanical systems (“MEMS”) microphones are used in a variety of fields to detect sound waves. Conventional MEMS microphones utilize a transduction method based on electrostatic sensing. In particular, one type of MEMS microphone uses a flexible moving diaphragm that forms one plate of a capacitor. The diaphragm moves relative to a fixed counter electrode that forms an opposite plate of the capacitor. As sound waves interact with the flexible diaphragm, the distance between the diaphragm and the counter electrode changes, thereby modifying the voltage/charge across the capacitor. Typically, a control circuit detects the changing voltage/charge across the capacitor and converts the detected voltage/charge into an electrical signal that is representative of the detected sound waves.
Diaphragm-type MEMS microphones are unsuitable for some applications, because the microphones exhibit viscous losses and thin-film squeezing damping, both of which negatively affect the detection of sound waves and decrease the signal to noise ratio (“SNR”) and the sensitivity of the microphone. Moreover, diaphragm-type MEMS microphones are susceptible to particles landing on the diaphragm, which can result in shorts and leakage paths between the diaphragm and the counter electrode, which may interfere with proper detection of the sound waves by the control circuit.
Another type of MEMS microphone is a piezoelectric-based MEMS microphone. The typical piezoelectric-based MEMS microphone utilizes a transduction method based on the piezoelectric effect. For example, the typical piezoelectric-based MEMS microphone includes a piezoelectric body connected to two electrodes. The piezoelectric body is vibrated by sound waves, which cause the piezoelectric body to deform. According to the piezoelectric effect, the deformation of the piezoelectric body produces a net charge between the electrodes that is detected by a control circuit, and which corresponds to the detected sound waves.
The typical piezoelectric-based MEMS microphone is subject to intrinsic material losses in the piezoelectric material of the piezoelectric body. The material loss typically negatively affects the electrical impedance and the quality factor of the piezoelectric body and may also change or otherwise negatively affect the resonance frequency of the MEMS microphone, thereby resulting in a microphone that degrades in performance over time.
What is needed, therefore, is a MEMS microphone that overcomes at least some of the shortcomings of known MEMS microphones.